The Ministry of Justice (MoJ) is facing calls to scrap and rewrite the policy on restraining minors after it emerged that a report it commissioned found some authorised techniques could kill children or leave them disabled.

An internal risk assessment of restraint techniques, obtained by the Guardian, shows that certain procedures approved for use against non-compliant children carry a 40% or 60% chance of causing injuries involving the child’s airway, breathing or circulation, the consequences of which could be “catastrophic”.

The undated report defines catastrophic injury as “death or permanent severe disability affecting everyday life” and ranks the chances of it occurring in each approved restraint technique out of five. It found that 28 of the 66 scenarios assessed had at least a 40% chance of causing that level of harm.

The medical review of restraints applied to children in young offender institutions (YOIs) and secure training centres (STCs) was completed for the government by an independent medical adviser and made public for the first time after a freedom of information (FoI) request.

Carolyne Willow, the director of children’s rights charity Article 39, who obtained the documents, said: “What kind of society trains adults to use restraint techniques that could cause children to die or be severely disabled?

“Techniques assessed as potentially causing catastrophic harm to children, with a likelihood of more than never, should be immediately struck out of the manual.

“Children have reported feeling terror, panic, desperation, shame, humiliation and anger during restraint. These testimonies from children, together with the appalling restraint-related deaths … should remain uppermost in our minds. We need a completely fresh start with policy and practice led by experts in looking after children.”

Willow has been engaged in a long-running legal battle for greater transparency in the government’s approach to youth restraints. The risk assessment forms part of a broader manual on restraint techniques, known as the MMPR (pdf), which has only been released to the public in redacted form, but the full version of which is the subject of an FoI appeal launched by Willow that could be taken to the court of appeal.

The MoJ refused to make the syllabus publicly available on the grounds that some of the techniques in the manual are similar to those used in adult prisons, and its release could undermine the safety of prisoners and guards across the justice system.

However, it released the risk assessment table for children being restrained using MMPR in detention or being taken to detention, which also includes children being deported. An MoJ spokesman said restraints were intended to be used as a last resort to prevent detainees harming themselves or others, to prevent damage to property and to prevent escape, but critics have warned that the guidelines are open to misuse.

He added: “Every incident of restraint is reviewed by the establishment. This includes looking at whether any lessons can be learned for the future. We have introduced a new system designed to minimise the use of physical restraint in youth custody and we are committed to improving practice further.”

YOIs take young people up to the age of 20. In March this year, there were 882 children (under-18s) in custody in England and Wales, and 4,668 18- to 20-year-olds. The three STCs in England and Wales have a combined capacity of about 270.

In 2015, there were 28 incidents of restraint per 100 children, which was up from 10 per 100 children in 2010. There were 429 injuries to children reported as a result of restraint in 2015.

Among the techniques identified as dangerous in the risk assessment is moving a child through a doorway while wearing a waist restraint belt, for which there is a three out of five chance of suffering breathing difficulties resulting in catastrophic harm.

The independent medical adviser also found that head holds, the mandibular angle technique (which involves applying upward pressure below the jaw, near the ear), moving a child under restraint, a figure of four leg hold in prone position, and strip-searching a child in prone position or on their side all had a two in five likelihood of resulting in grave harm.

Moving a non-compliant child off transport, restraining a child on the ground, a figure of four arm hold to an inverted wrist, thumb flexion, moving a child under restraint “whilst negotiating stairs”, rigid bar handcuffs, straight arm hold to an inverted wrist also all had a three out of five chance of resulting in fracture or dislocation and ligament or tendon damage.

The assessment acknowledges that such injuries require hospital attendance or admission but rates them only as “minor” or “moderate”.

The Youth Justice Board (YJB) says officers receive “detailed medical advice on the physiological aspects of the youngest age range to make sure that the use of force is minimised, used only when necessary, and, when it is used, that the risk of injury to young people is minimised”.

Staff are trained in the use of MMPR by experts, and understanding is assessed through a written test and scenario-based exercise. The manual states that they “must be able to demonstrate the required level of understanding before being authorised to use MMPR in an operational environment”. Staff receive a minimum of one day’s refresher training every six months.

Willow has fought for children’s rights in custody since the death of 15-year-old Gareth Myatt in 2004. He was being restrained after refusing to clean a sandwich toaster at Rainsbrook secure training centre. Myatt, who weighed 40kg (six and a half stone) and stood 1.47m (4ft 10in) tall, was forced into a sitting position by three G4S officers, his upper body bent towards his thighs and knees. The officers ignored Myatt’s cries that he could not breathe and was going to defecate, which he did before vomiting. One restraining officer was 1.85m tall (6ft 1in) and 90kg (14st). When Myatt told the officers he could not breathe, one replied: “Well, if you are shouting, you can breathe.”

His death was later found at an inquest to be accidental. The restraint technique used, known as the seated double embrace, was withdrawn two months after he died.

In granting the FoI request for the risk assessment, the MoJ stressed: “If a technique scores highly in the consequences section, it does not mean that this injury will occur, as it is scored with the worst possible outcome in mind.”

It added: “To date there have been no recorded serious injuries or warning signs recorded from the escort services.”

Dave Hill, president of the Association of Directors of Children’s Services, said: “Department for Education guidance is absolutely clear in that restraint should only be used where absolutely necessary and should be reasonable and proportionate to the circumstances. Restraint that deliberately inflicts pain should never be used on children.”

A spokeswoman for the YJB said: “The safety and welfare of the young people in the youth justice system is of paramount importance to the YJB. MMPR was devised by a team of experts who extensively researched and carefully considered the best ways to keep children in custody, and the staff looking after them, safe. Restraint should only ever be used as a last resort with staff using their relationships with the children in their care to manage their behaviour. Her Majesty’s Inspectorate of Prisons’ thematic on MMPR published last year concluded that MMPR was a good system but it needed more time to be embedded.”